1. Field of the Invention
The present invention relate to a negative active material, a method of preparing the negative active material, and a lithium battery including the negative active materials, and more particularly, to a negative active materials with an improved cycle characteristic and capacity retention rate, a method of preparing the negative active material, and a lithium battery including the negative active material.
2. Description of the Related Art
Lithium batteries used in portable electronic devices for information communication, such as personal digital assistances (PDAs), mobile phones, or notebook computers, electric bicycles, electric vehicles, or the like have discharge voltages that are at least two times greater than that of existing batteries. Accordingly, lithium batteries exhibit high energy densities.
A typical lithium battery includes a positive electrode, a negative electrode, and either an organic electrolytic solution or a polymer electrolyte filling the space between the positive and negative electrodes. The positive and negative electrodes each include an active material that allows lithium ions to be intercalated and deintercalated. In this structure, when lithium ions are intercalated and deintercalated in the positive and negative electrodes, oxidation and reduction reactions occur, and thus, electrical energy is generated.
The positive active materials of positive electrodes for lithium batteries may be, for example, oxides of lithium and a transition metal that allow intercalation of lithium ions, such as a lithium cobalt oxide (LiCoO2), a lithium nickel oxide (LiNiO2, or a lithium nickel cobalt manganese oxide (e.g., Li(NiCoMn)O2 or Li(Ni1-x-yCoxMny)O2 (0<x<1, 0<y<1, and 0<x-+y<1).
Research into negative active materials of negative electrodes for lithium batteries that allow intercalation and deintercalation of lithium ions, such as various types of carbonaceous materials including artificial and natural graphite and hard carbon, and non-carbonaceous materials such as Si with a high capacity, has been conducted.
However, non-carbonaceous materials such as Si repeatedly undergo volumetric expansion and contraction during intercalation and deintercalation of lithium ions, and thus, a negative electrode including such a non-carbonaceous material has an unstable structure and decreased cycle life. To address problems with carbonaceous and non-carbonaceous active materials, research into Si-based alloys has been conducted.
A Si-based alloy generally has an active Si phase and an inactive matrix phase. However, due to such two phases, Si-based alloys other than some non-Si-based alloys having a one-matrix phase have lower capacities than silicon.
Therefore, there is still a need to develop a negative active material such as a Si-based alloy with improved cycle characteristics and an improved capacity retention rate, a method of preparing the negative active material, and a lithium battery including the negative active material.